The invention relates to the field of flat panel displays, and in particular, to the manufacture of opaque rib structures for plasma addressed liquid crystal (PALC) displays.
Flat panel displays, e.g., liquid crystal displays, are known. Recently, the use of plasma channels to address a liquid crystal display (LCD) has become known. For example, U.S. Pat. Nos. 4,896,149, 5,036,317, 5,077,553, 5,272,472, 5,313,223, the disclosures of which are all hereby incorporated by reference, each disclose such a structure. This type of display technology provides an active addressing matrix suitable for high-line-count displays, and is a competitive alternative to the known thin-film transistor (TFT) active matrix approach.
These plasma channel panels are also referred to herein as plasma addressed liquid crystal (PALC) displays. This type of plasma display panel is generally formed of two parallel substrates separated from each other to form a discharge space between the substrates, which is filled with a discharge gas, such as a mixture of helium, neon and xenon. The inner-facing surface of each of the substrates bears a pattern of spaced parallel electrodes, with the electrodes on one substrate being oriented, for example, in a direction orthogonal to the direction of the electrodes on the other substrate. The electrode bearing surfaces of the substrates are typically covered with a dielectric layer, and red, green and blue phosphors are separately located in discrete areas on the internal surface of the dielectric layer on one of the two substrates. The dielectric layers are generally lead-based glass frits fired between 500 and 600xc2x0 C., depending on their formulation and the level of uniformity required. The displayed picture is produced by plasma discharges which are induced locally in the gas by applying a suitable voltage between the electrodes of one substrate and the electrodes of the other substrate. Ultraviolet light emitted locally by the gas discharge induces luminescence of the neighboring phosphors.
A PALC display relies on the highly non-linear electrical behavior of a relatively low pressure (e.g., 10 to 100 Torr) gas, e.g., He, confined in parallel channels. A cross section of a portion of a PALC display 100 is shown in FIG. 1. A pair of parallel electrodes 101A (anode) and 101C (cathode) is deposited in each channel 102 on a rear glass plate 101G, for example, forming the bottom of the channels, and a very thin dielectric sheet 103, e.g., a glass micro-sheet of about 50 xcexcm thickness, forms the top of the channels 102. A liquid crystal layer 104 on top of the micro-sheet 103 is the optically active portion of the display 100. A cover sheet 105, e.g., a passive glass plate of about 1.1 mm, with transparent conducting electrodes, e.g., made from indium-tin oxide (ITO), running perpendicular to the plasma channels 102, lies on top of the liquid crystal 104. Conventional polarizers 106, color filters 107, and back lights 108, like those found in other conventional liquid crystal displays, are also commonly used, as illustrated.
When voltages are applied to the transparent electrodes, since there is no ground plane, the voltages are divided among the liquid crystal, the micro-sheet, the plasma channel, and any other insulators intervening between the transparent electrode and whatever becomes the virtual ground. As a practical matter, this means that if there is no plasma in the plasma channel, the voltage drop across the liquid crystal will be negligible, and the pixels defined by the crossings of the transparent electrodes and the plasma channels will not switch. If, however, a voltage difference sufficient to ionize the gas is first applied between the pair of electrodes in a plasma channel, a plasma forms in the plasma channel so that it becomes conducting, and constitutes a ground plane. As a result, for pixels atop this channel, the voltages will be divided between the liquid crystal and the micro-sheet only. This places a substantial voltage across the liquid crystal and causes the pixel to switch. Igniting a plasma in the channel causes the row above the channel to be selected. Because the gas in the channels is non-conducting until a well-defined threshold voltage between the electrode pair is reached, the rows are extremely well isolated from the column voltages unless selected. This high non-linearity allows large numbers of rows to be addressed without loss of contrast.
In order to avoid luminous cross-talk between neighboring regions and improve the contrast in such displays, opaque barrier ribs 110 are disposed on at least one of the substrates (typically the rear one) forming electrically insulated discharge cells. The barrier rib structure is typically periodic with a pitch of, for example, from 200 xcexcm to 400 xcexcm, depending on the panel resolution. These ribs are, for example, about 30-100 xcexcm wide and 100-200 xcexcm thick (i.e., high).
Alternatively, a closed cell design has been employed having square cells which are about 200-400 xcexcm on each side. The xe2x80x9cribsxe2x80x9d which form these square cells are about 30 xcexcm to 70 xcexcm wide and about 30 to 200 xcexcm high. Plasma panels of this type are described, for example, in U.S. Pat. No. 4,853,590, as well as Japanese Patent Application Nos. J04255638 and J04075232. The networks of parallel barrier ribs mentioned above delimit columns of pixels which can be addressed independently. The two perpendicular networks of electrodes allow ionization of the gas at the selected pixels. The ultraviolet radiation emitted by the ionized gas causes the excitation of areas of phosphorescent products associated with said pixels according to the configuration of an image which is to be displayed.
In the past, the barrier ribs have typically been made either by a silk-screening method, or by sandblasting from a deposited layer of frit. Related co-pending U.S. application Ser. No. 08/820,206, referenced above, discloses micro-molding processes for making the barrier ribs. One disadvantage associated with these micromolding methods, particularly when depositing opaque rib materials, is the possibility of depositing a thin film of opaque material on the glass substrate between rib structures. Conventional screen printing and photolithography-based methods typically avoid residual film formation between the ribs. However, for low cost processes in which it is desirable to limit the number of printing steps, screen printing and photolithography-based methods are limited to producing low thickness ribs, e.g., about 20 microns. Conventional methods, however, typically use solvent based materials which can cause difficulties in maintaining rib shapes, particularly high aspect ratio ribs, usually requiring additional consolidation steps to maintain rib shape. Accordingly, a need exists for improved methods of manufacturing opaque rib structures for PALC displays.
This invention provides novel methods for making opaque rib structures for flat panel displays that utilize micro-molding techniques but that do not leave a residual film of opaque material between the rib structures. The invention also provides improved micromolding methods that result in improved structures and lower manufacturing costs.
According to an aspect of the invention, a method of manufacturing opaque rib structures for use in a flat panel display, such as a plasma addressed liquid crystal (PALC) display, includes providing a substrate and an intaglio collector having cavities formed in its surface complimentary to the desired size and spacing of the barrier ribs. A hardenable glass paste which includes a glass frit and a hardenable, settable or curable medium (hereinafter referred to collectively as xe2x80x9ccurablexe2x80x9d), e.g., an ultra-violet sensitive medium, is provided into the collector cavities to define rib structures. Useful curable media should be micromoldable and easily removed by burning, and include both thermoplastic and thermosetting materials. However, thermoplastic materials are generally preferred. The rib structures are transferred from the collector to a surface of the substrate while being at least partially hardened or cured, by exposure to ultra-violet light, for example. The curable medium, e.g., the ultra-violet sensitive medium, may then be removed from the rib structures on the substrate surface by performing a burn-out generating porosity in the rib structures. The substrate having porous rib structures may then be dipped into a solution containing an opaque pigment which is absorbed into the rib structures, rinsed in a suitable rinsing solution, such as water and alcohol, to remove excess pigment, and fired.
According to another aspect of the invention, an alternative method of manufacturing opaque rib structures for use in a flat panel display, such as a plasma addressed liquid crystal (PALC) display, includes: (a) forming a temporary mask on portions of a surface of a substrate; (b) depositing a calibrated layer, i.e., a layer having a uniform thickness within a close tolerance, e.g., xc2x110%, of a hardenable glass paste containing glass frit, a curable medium, e.g., a thermoplastic or thermosetting medium, and at least one opaque pigment blended therein, over the substrate and temporary mask; (c) micro-molding rib structures of the glass paste by application of an intaglio plate or roll to the coated substrate surface and curing the curable medium; (d) removing any residual layer of opaque paste material present on the temporary mask between the rib structures; (e) removing the temporary mask from the substrate; and (f) firing the substrate with micro-molded rib structures thereon. Useful curable media should be micromoldable and easily removeable by burning, and are preferably thermoplastic.
According to another aspect of the invention, a further alternative method of manufacturing opaque rib structures for use in a flat panel display, such as a plasma addressed liquid crystal (PALC) display, includes extruding a glass paste having a curable medium onto a surface of a substrate in a pattern defining ribs, and firing the substrate with glass paste rib pattern thereon. As in the other embodiments, useful curable media should be easily removable by burning, and are preferably thermoplastic. The glass paste may be impregnated with an opaque pigment, or may be made porous, dipped and rinsed, as summarized above with respect to the first alternative methods.
These and other aspects of the invention will become apparent from the detailed description set forth below.